Which one will be best overall?  Including graphics, gameplay, cars available, etc.

Forza 3 will be amazing, good graphics, gameplay and selection. But GT5 is just going to be an absolute beast.

GT5 by far.

I don't know enough about Forza to draw a meaningful comparison, but I do know GT5 is going to be insane.

GT5 will be the best.

And People at other websites(game...trailers) think Forza 2 is better,

GT5 with out isitation

GT5 by miles.

GT5 will be the best racing game ever.

I would have liked to see some Lamborghinis and Porsches in Gt5 though, I still have hopes!

GT5 by miles, at least for me!
I've played both Forza and Forza 2, and I concluded that they're probably the best racing games on their respective platforms.
I did not feel that they're superior to GT, though, I actually still place GT on the top of the racing list across platforms.
As a sidenote; its a tad unfair to compare Forza to GT4 like many would do, it should rather be compared to GT5 when it arrives.
And next to that, I don't see how it could win, knowing the amazing job Polyphony did on the PS2 one can only imagine what they'll accomplish on the next generation of hardware! Can't wait for this title!